In a direct injection internal combustion engine, a fuel injector is provided to deliver a charge of fuel to a combustion chamber prior to ignition. Typically, the fuel injector is mounted in a cylinder head with respect to the combustion chamber such that its tip protrudes slightly into the chamber in order to deliver a charge of fuel into the chamber.
One type of fuel injector that is particularly suited for use in a direct injection engine is a so-called piezoelectric injector. Such an injector allows precise control of the timing and total delivery volume of a fuel injection event. This permits improved control over the combustion process which is beneficial in terms of exhaust emissions.
A known piezoelectric injector 2 and its associated control system 4 are shown schematically in FIG. 1. The piezoelectric injector 2 is connected to an injector drive circuit 6 by way of first and second power leads 8, 10. The piezoelectric injector 2 includes a piezoelectric actuator 12 that is operable to control the position of an injector valve needle 14 relative to a valve needle seat 16. The piezoelectric actuator 12 includes a stack 18 of piezoelectric elements that expands and contracts in dependence on a differential voltage supplied by the injector drive circuit 6.
The axial position, or ‘lift’, of the valve needle 14 is controlled by varying the differential voltage across the actuator 12. By application of an appropriate voltage differential across the actuator 12, the valve needle 14 is either caused to disengage the valve seat 16, in which case fuel is delivered into an associated combustion chamber (not shown) through a set of nozzle outlets 20, or is caused to engage the valve seat 16, in which case fuel delivery through the outlets 20 is prevented.
Piezoelectric injectors 2 are typically grouped together in banks. As described in EP1400676, each bank of piezoelectric injectors 2 has its own drive circuit 6 for controlling operation of the piezoelectric injectors 2. The circuitry typically includes a power source, which steps-up the voltage generated by a nominal-voltage power source (e.g., an automobile battery) from its nominal voltage level (e.g., 12 Volts) to a higher voltage, and storage capacitors for storing charge and, thus, energy. The higher voltage is applied across the storage capacitors, which are used to power the charging and discharging of the piezoelectric injectors 2 for each injection event.
As shown in FIG. 1, piezoelectric injectors 2 are controlled by an injector control unit 22 (ICU) that forms an integral part of an engine control unit 24 (ECU). The ICU 22 typically comprises a microprocessor 26 and memory 28. The ECU 24 monitors a plurality of engine operating parameters 30, and calculates an engine power requirement signal (not shown), which is input to the ICU 22. Examples of the engine operating parameters 30 include engine speed, driver torque demand, manifold inlet pressure and manifold inlet temperature. In turn, the ICU 22 calculates a required injection event sequence to provide the required power for the engine and operates the injector drive circuit 6 accordingly.
Each piezoelectric injector 2 is operable to deliver one or more injections of fuel within an injection event sequence. For example, an injection event sequence may include one or more so-called ‘pre-’ or ‘pilot’ injections, one or more main injections, and one or more ‘post’ injections. The use of several such injections within an injection event sequence can increase the combustion efficiency of the engine in order to meet emissions, fuel consumption and NVH (Noise Vibration Harshness) targets.
A problem can occur when an engine is run at high speeds and/or loads, wherein the ICU may calculate certain injection event sequences that can overload the power source as it provides power to the injector drive circuit 6. If this occurs, the injector drive circuit 6 is unable to provide sufficient power to operate the piezoelectric injectors 2 according to the required injection event sequence. This may cause the piezoelectric injectors 2 to deliver less fuel than is required, which in turn may result in an undesirable and unexpected loss of power to the vehicle engine, or the engine misfiring.
It is an object of the present invention to provide an improved method of operating fuel injection equipment, which prevents the aforementioned problem from occurring. The invention also aims to provide an improved apparatus for operating fuel injection equipment.